The use of photoresists in the course of manufacture of printed circuit boards and the like, is well-known and well-established. Basically, the process comprises forming a layer of a photoresist material (or, simply, "resist") upon an appropriate substrate; patternwise exposing the layer to actinic radiation; and then "developing" the exposed layer by removal of uncured portions of the layer not exposed to the radiation. In particular, a photoresist technique is used to produce patterned images for selective etching of a metal substrate, for the plating of a metal upon a substrate, and for the application of a solder to a substrate.
Much emphasis is placed on efforts to avoid the use of organic solvents in the preparation and use of photoresists. Increasingly, photoresist systems based on water are being used and can be supplied in film or liquid form. See, for example U.S. Pat. Nos. 4,100,047, 4,564,580 and 4,133,909 which describe aqueous coatings; and U.S. Pat. Nos. 3,953,309 and 4,361,640 which teach compositions that can be developed with aqueous solutions. Another aqueous processable photopolymerizable composition is described in U.S. Pat. No. 4,510,230, disclosing organic acids employed within an acid binder photopolymer composition to decrease scum or stain on a copper surface.
Aqueous emulsions, while more environmentally acceptable, are often plagued with stability problems associated with the processing and applications of the emulsions. In processing of the aqueous emulsions, high shear rates are frequently used in the emulsification process, especially in the case of direct emulsification of previously synthesized polymers. In many cases the emulsions are then concentrated by evaporation of water and/or organic solvents, often under vacuum and with heat and mechanical shear stress applied (e.g. wiped film evaporators). In the cases where organic solvents are emulsified along with other water-insoluble components, the evaporation process itself can be mechanically stressful as the solvent vapors must pass through the surfactant boundary, thus disrupting the stabilizing forces. Finally, the emulsions are frequently formulated with other additives using high shear rates to mix the components. Examples range from pigment grinding, a very high shear process, to simple admixing of water soluble additives. Often times these additives, examples of which include water-dispersable thickeners and organic cosolvents, can increase emulsion sensitivity to mechanical or thermal stress.
In application, aqueous emulsions may be applied as coatings to substrates using a variety of methods which may involve a range from a very low shear to a very high shear process. Among the higher shear processes are spraying and roll-coating processes. Roll-coating can be especially stressful due to longer exposure time of the emulsion composition to high shear (i.e., in the roller nips). For practical utility, it is imperative that the emulsion composition does not degrade or coagulate under such conditions.
Optimization of surfactants has traditionally been the approach taken to overcome shear stability problems in aqueous emulsion systems. However, high levels of surfactants are often required to provide stability, but excessive levels of surfactants can cause additional problems. These additional problems include foam stabilization during high shear process which can lead to the formation of bubbles or pinholes in the final film product. Excessive levels of surfactants can also lead to adhesion failure at the substrate/coating interface and sensitivity of the resulting coating to water or humidity.
Consequently, there exists a need for photoresist materials, and processes associated therewith, which meet the requirements for a waterborne, liquid applied contact imageable photoresist and yet possess mechanical or shear stability while minimizing the need for surfactants.